drm_hwcomposer: Move HardwareSupportsLayerType into Backend

The HardwareSupportsLayerType function is used only for display validation
which is implemented in the generic backend.

Signed-off-by: Matvii Zorin <matvii.zorin@globallogic.com>
diff --git a/backend/Backend.h b/backend/Backend.h
index 821994f..6aca247 100644
--- a/backend/Backend.h
+++ b/backend/Backend.h
@@ -34,6 +34,7 @@
                              DrmHwcTwo::HwcLayer *layer);
 
  protected:
+  bool HardwareSupportsLayerType(HWC2::Composition comp_type);
   uint32_t CalcPixOps(const std::map<uint32_t, DrmHwcTwo::HwcLayer *> &z_map,
                       size_t first_z, size_t size);
   void MarkValidated(std::map<uint32_t, DrmHwcTwo::HwcLayer *> &z_map,